LT3757 datasheet(21/28 Pages) AD

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LT8392

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 12 shows the implementation of external shut-

down control while still using the UVLO function. The

NMOS grounds the EN/UVLO pin when turned on, and

puts the LT8392 in shutdown with quiescent current less

than 2µA.

Figure 13. Programming Input Current Limit

LT8392

ISP

8392 F13a

ISN

RIS

TO DRAIN OF

SWITCH A

FROM POWER

INPUT

LT8392

ISP

8392 F13b

ISN

RIS

TO DRAIN OF

SWITCH A

FROM POWER

INPUT

(13a)

(13b)

When VCTRL is larger than 1.35V, the current threshold

is regulated to:

IIS(MAX) =

50mV

RIS

The CTRL pin should not be left open (tie to VREF if not

used). The CTRL pin can also be used in conjunction with

a thermistor to provide overtemperature protection for

the output load, or with a resistor divider to VIN to reduce

output power and switching current when VIN is low.

The presence of a time varying differential voltage rip-

ple signal across the ISP and ISN pins at the switching

frequency is expected. If the current sense resistor RIS

is placed between power input and input bulk capacitor

(Figure 13a), or between output bulk capacitor and sys-

tem output (Figure 14a), a filter is typically not neces-

sary. If the RIS is placed between input bulk capacitor

and input decoupling capacitor (Figure 13b), or between

output decoupling capacitor and output bulk capacitor

(Figure 14b), a low pass filter formed by RF and CF is

recommended to reduce the current ripple and stabilize

Figure 12. VIN Undervoltage Lockout (UVLO)

LT8392

VIN

RUN/STOP

CONTROL

(OPTIONAL)

8392 F12

EN/UVLO

GND

Programming Input or Output Current Limit

The input or output current limit can be programmed by

placing an appropriate value current sense resistor, RIS, in

the input or output power path. The voltage drop across

RIS is (Kelvin) sensed by the ISP and ISN pins. The CTRL

pin should be tied to a voltage higher than 1.35V to get

the full-scale 50mV (typical) threshold across the sense

resistor. The CTRL pin can be used to reduce the current

threshold to zero, although relative accuracy decreases

with the decreasing sense threshold. When the CTRL pin

voltage is between 0.3V and 1.15V, the current limit is:

IIS(MAX) =

VCTRL −0.25V

20 •RIS

When VCTRL is between 1.15V and 1.35V the current limit

varies with VCTRL, but departs from the equation above

by an increasing amount as VCTRL increases. Ultimately,

when VCTRL is larger than 1.35V, the current limit no lon-

ger varies. The typical V(ISP-ISN) threshold vs VCTRL is

listed in Table 2.

Table 2. V(ISP-ISN) Threshold vs VCTRL

VCTRL (V)

V(ISP-ISN) (mV)

1.15

1.20

47.2

1.25

1.30

49.8

1.35

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